1. Field of the Invention
This invention relates to millimeter wave oscillators and, more specifically, to a gallium arsenide travelling-wave transistor oscillator having extended oscillation frequency.
2. Summary of the Prior Art
Monolithic gallium arsenide integrated circuit technology has been found suitable for system applications at millimeter wave frequencies because of the size advantage thereof and ease of control of parasitics. Typical applications are single chip receivers having a mixer, a local oscillator and an IF amplifier on the same chip.
In the prior art, the oscillator has been provided by use of a discrete gallium arsenide field effect transistor in the oscillator circuit. These gallium arsenide field effect transistors displayed frequency limitations of operation due to their intrinsic properties. It is readily apparent that it is desirable to extend the frequency range of oscillators as far as possible. Attempts to use other types of devices such as GUNN or IMPATT diodes have provided results inferior to gallium arsenide FETs because of the lower efficiency thereof due to thermal problems. Furthermore gallium arsenide FETs have been more desirable than the GUNN or IMPATT devices because the semi-insulating property of the FET substrate is more compatible with monolithic integration with other types of components.
Conventional circuit feedback techniques, such as common-gate, common-drain and the like have generally been used in conjunction with gallium arsenide FET oscillators at frequencies below 20 gigahertz. These oscillators have generally displayed a maximum oscillator frequency, f.sub.max, of approximately 40 gigahertz. Circuit parasitics of such devices have generally limited the operating frequency to about one-half of f.sub.max or about 20 gigahertz as mentioned hereinabove. In order to extend the upper oscillation frequency limit in the prior art, harmonic extraction of a fundamental frequency oscillator has generally been used, but only at the expense of greater circuit complexity and concommitant increases in cost.